When converters are used to feed electrical systems, for example three-phase machines or three-phase mains systems, the current also contains, in addition to the desired first order positive phase-sequence system, n-th order systems which are undesirable, for example because of the additional losses that they cause.
If one wishes to suppress n-th order electrical systems, then a corresponding voltage system must be produced for this purpose, which is supplied, as pilot control for manipulated variables, to a converter-fed system.
The article "Koordinatentransforationen fur MehrgroBen-Regel-systeme zur Kompensation und Symmetrierung von Drehstrom-netzen" (Coordinate Transformations for Multi-Variable Control Systems for Power Factor Correction Balancing of Three-Phase Mains System) by W. Meusl and H. Waldmann, printed in the German journal "Siemens Forsch.--und Entwickl.--Ber.", (Siemens Research and Development Reports) Volume 6, 1977, No. 1, pages 3 to 12, describes a control system, also called a multi-variable control sytem, for a solid-state power factor corrector which is used in a three-phase arc furnace. This control system always drives thyristors of the solid-state power factor corrector such that the reactive current load on the mains system is as low as possible, as constant as possible and such that the load on the mains system overall is as balanced as possible. In the representation using symmetrical components, this task is defined as follows:
The reactive element of the positive phase-sequence system of the mains current should be as small and constant as possible. At the same time, the negative phase-sequence system should be as small as possible, overall.
Since there is no neutral connection in this arrangement, the zero phase-sequence system is always equal to zero.
The article summarizes the most important relationships between a number of component systems, and illustrates these relationships in matrices in Table 1. When phase currents ((R,S,T) components) are converted into balanced components ((0, 1, 2) components), a (0, .alpha., .beta.) system can also be used, as shown in Table 1. Implementation of direct conversion requires greater equipment complexity.
In this multi-variable control system for a solid-state power factor corrector, analysis of the furnace currents produces the n-phase and reactive current components of the positive phase-sequence system, characterized by index 1, and of the negative phase-sequence system, characterized by index 2. The n-phase current components of the positive phase-sequence system are not processed any further. The reactive current components of the positive phase-sequence system and both components of the negative phase-sequence system are intended to be canceled out by the power factor correction system. These components are used as reference variables in this control system. The control variables are determined from the currents on the mains system side of the power factor correction system, using the same method as for the reference variables. Since this control system essentially has to process only reference variable changes, pilot control is used. The outputs of the positive phase-sequence system reactive current component controller, of the. negative phase-sequence system in-phase current components controller and of the negative phase-sequence system reactive current component controller are transformed into phase variables, which are then converted into control signals for the thyristors of the power factor correction system. This control system is used to generate control signals for each phase of the power factor correction system, as a result of which specific values are obtained for positive phase-sequence and negative phase-sequence systems on the mains side.
This control system influences only the components of the first order positive phase-sequence and negative phase-sequence system, and not n-th order electrical systems.
In the case of a direct converter with an open circuit, harmonics can occur despite sinusoidal control of the phase voltages if the back e.m.f. produced by the load, as a rule a motor, has a waveform which is not sinusoidal. Special synchronous machines produce a back e.m.f. which may contain a high proportion of 3rd harmonics. In the case of the salient pole machine which is used very frequently, it is possible, by appropriate design of the airgap, to achieve the production of a very good sinusoidal back e.m.f. on no load, but, when loaded, field distortion occurs as a result of the pole axis being shifted with respect to the axis of the rotating field, and causes corresponding harmonics of the back e.m.f. which is produced.